Chapter 8 DNA & RNA

#1. DNA Structure (an overview) DNA has three main components 1. deoxyribose (a pentose sugar) 2. base (there are four different ones) 3. phosphate

#2. The Bases They are divided into two groups Pyrimidines and purines Pyrimidines (made of one 6 member ring) Thymine Cytosine Purines (made of a 6 member ring, fused to a 5 member ring) Adenine Guanine The rings are not only made of carbon (specific formulas and structures are not required for IB)

Nucleotides Units that make up DNA molecule Made of three parts 5 carbon sugar (deoxyribose) Phosphate group Nitrogen bases

#3. Nucleotide Structure Nucleotides are formed by the condensation of a pentose sugar, phosphate and one of the 4 bases The following illustration represents one nucleotide

#3. Nucleotide Structure Nucleotides are linked together by covalent bonds called phosphodiester linkage

#4. DNA Double Helix and Hydrogen Bonding Made of two strands of nucleotides that are joined together by hydrogen bonding Hydrogen bonding occurs as a result of complimentary base pairing Adenine and thymine pair up Cytosine and guanine pair up Each pair is connected through hydrogen bonding Hydrogen bonding always occurs between one pyrimidine and one purine

#4. DNA Double Helix and Hydrogen Bonding Complimentary base pairing of pyrimidines and purines

#4. DNA Double Helix and Hydrogen Bonding

Watson and Crick later discovered what held the two strands together Hydrogen bonds could form between certain nitrogen bases and provide enough force to hold the two strands together Hydrogen bonds could only form between certain base pairs adenine and thymine and guanine and cytosine This principal is called Base pairing This explains Chargaff’s Rule

#4. DNA Double Helix and Hydrogen Bonding Adenine always pairs with thymine because they form two H bonds with each other Cytosine always pairs with guanine because they form three hydrogen bonds with each other

#5. DNA Double Helix The ‘backbones’ of DNA molecules are made of alternating sugar and phosphates The ‘rungs on the ladder’ are made of bases that are hydrogen bonded to each other

#6. Antiparallel strands 5’ 3’ The strands run opposite of each other. The 5’ end always has the phosphate attached. 5’ 3’